4-Bromo-o-xylene is useful in the preparation of diphenyl ethers. Illustrative such ethers are 3,3',4,4'-tetramethyldiphenyl ether, 1,4-bis(3,4-dimethylphenoxy) benzene, and the like. 4-Bromo-o-xylene is also useful as a starting reagent in some syntheses of riboflavin (vitamin B.sub.2).
4-Bromo-o-xylene can be prepared from the reaction of 3,4-dimethylaniline with CuBr.sub.2 NO. This reaction proceeds through a diazonium intermediate, but such a preparation procedure is relatively costly, and is impractical on a large scale.
A route for commercial scale preparation of 4-bromo-o-xylene involves the addition of bromine (Br.sub.2) to o-xylene. However, when one follows the solvent-less preparation procedure described in "Organic Syntheses", Vol. 3, pp 138, 139 (1955): ##STR1## wherein the mole ratio of bromine to o-xylene is 0.875 and uses a temperature of 0.degree. to -5.degree. C., it is found by analysis that this procedure produces a 75:25 mixture of 4-bromo-o-xylene to 3-bromo-o-xylene in a 95 percent yield.
These two bromo-o-xylene isomers are extremely difficult to separate because they have virtually the same boiling point (214.degree. C. for 4-bromo-o-xylene compared to 215.degree. C. for 3-bromo-o-xylene). These isomers also cannot be separated by crystallization. They are also difficult to separate by gas chromatography (G.C.). A capillary G.C. column with cyanopropyl methyl silicone coating is needed to separate these isomers. It is therefore desirable to synthesize 4-bromo-o-xylene in as high a purity as possible to minimize the quantity of admixed 3-bromo-o-xylene present.
J. P. Canselier has reported (Bull.Soc.Chim. France, 1972, pp. 762-764, and Bull.Soc.Chim. France, 1971, pp. 1785-1788) achieving a 93.4:6.5 mixture of 4-bromo-o-xylene to 3-bromo-o-xylene by reacting a mixture of bromine, and o-xylene in SO.sub.2 solution. The procedure for achieving a 93.4:6.5 mixture of 4-bromo-o-xylene to 3-bromo-o-xylene is not given but it is stated that it was run according to a process described for toluene, i.e., a mixture of bromine and sulfur dioxide is added to a very dilute mixture of the hydrocarbon at -40.degree. C. and in a darkened reactor. The resulting mixture is then agitated at -9.degree. C. for 3 hours before the sulfur dioxide is removed and the product isolated by distillation. His procedure is not suitable for commercial scale preparation of 4-bromo-o-xylene in high yields.
A process that produces 4-bromo-o-xylene in relatively high yield and in combination with a minimal amount of 3-bromo-o-xylene and other by-products would be commercially desirable. The present invention provides such a process.